1. Field of the Invention
This invention relates generally to asynchronous clock operation in optical transmission networks and more particularly to the deployment of asynchronous operation for an optical transport network (OTN) designed to operate asynchronously by means of mapping and remapping of client signal overhead or 10GbE signal protocol where the line rate, however, is maintained constant throughout signal transport.
2. Introduction
As used herein, the terms “rate” and “frequency” are to have the same meaning. Also, there is discussion about deployment of the invention herein in end terminals and intermediate nodes of transmission network, in particular as exemplified in optical transmission systems following in part the OTN protocol. As used herein, reference to “nodes (NEs)” is intended to cover both “intermediate nodes” of a network as well as “end terminals”. In this connection, end terminals may operate under an accurate crystal oscillator with a phase locked loop (PLL) circuit but, according to this invention, it is not necessary to employ suck crystal clocks at an intermediate node. Instead, a less expensive local reference clock may be deployed.
3. Description of the Related Art
The ITU-T G.709 entitled, “Interface for the optical transport network (OTN)”, a proposed international standard for the OTN architecture is intended to converge existing packet services, such as ATM, IP Ethernet, and TDM broadband services, such as SONET and SDH, transparently via the shortest possible stack onto a single network while providing enhanced signal amplification and networking function to all client services. Like SONET, the OTN architecture assumes that the transport function through the network is synchronous in the digital or electrical/electronic domain. Frames received at the ingress of a G.709 network node are to be retransmitted at the node at an identical frame rate. The G.709 specification specifies how to synchronously or asynchronously map, for example, the SDH STM-64 protocol data into G.709 OTU2 frames and visa versa at the ingress and egress of the OTN. The G.709 specification does not specify how to map IEEE 802.3ae 10 GbE protocol data into G.709 frames, whether done synchronously or asynchronously.
SONET employs a single timing approach which has a primary benefit of enabling the combining one or more data streams at a predetermined rate into higher data rate streams and extracting one or more data streams without demultiplexing the entire higher data rate stream. The G.709 protocol basically assumes that intermediate points of the network, i.e., optical cross-connects, optical add/drop multiplexers and the like, operate in a synchronous fashion, even if asynchronous mapping of data or overhead are deployed at the network ingress or egress. This results in added system costs (COGS) because it is necessary, for example, to provide accurate clocking at each node in order to “recapture” the clock of the original transmitter node. Accurate clocking entails the utilization of high cost, phase locked loop (PLL) circuitry and an expense crystal oscillator together with other required system components.
In a plesiochronous clocking system, each subsystem or node in the network may be designed to have its own local clock generation so that different subsystems are operating at, at least, slightly different clock frequencies. To accommodate the different frequencies, bit stuffing techniques are employed. Bit stuffing is well known in the art in many different technical disciplines. For some further background concerning plesiochronous clocking systems, including ways of handing clock differences with a combination phase lock loop/delay lock loop approach, see, for example, U.S. patent application to Tang et al., Publication No. 2002/0075980, published on Jun. 20, 2002 (U.S. patent application Ser. No. 10/029,709).
As indicated above, the standard G.709 digital wrapper, the payload signal can be mapped into the digital wrapper in two ways, synchronous and asynchronous mapping. In synchronous mapping case, the digitally wrapped signal frequency is exactly equal to the incoming payload signal frequency times a fixed overhead ratio (F-OHR). If the incoming payload signal frequency varies slightly, the digitally wrapped signal frequency will track that variation. In asynchronous mapping case, the digitally wrapped signal frequency is equal to the payload envelope frequency times a fixed overhead ratio (F-OHR). The payload envelope frequency is generated by the wrapper and is not frequency locked to the incoming payload signal. The frequency difference between the payload envelope and the incoming payload signal is accommodated by some kind of justification mechanism.
In both the conventional synchronous and asynchronous mapping cases, the digitally wrapped signal frequency is scaled with the payload signal frequency. The digitally wrapped signal would be running at a different frequency if the payload signal is at a different nominal frequency. For example, the nominal frequency for OC192 is 9.95328 Gbps with +/−20 ppm variation. If the OC192 payload signal happens to be 9.95328 Gbps+10 ppm, the synchronously mapped G.709 signal would be 9.95328 times the F-OHR of 255/237 Gbps+10 ppm. If this signal is mapped to G.709 asynchronously and the local reference frequency offset is −5 ppm, the asynchronously mapped G.709 signal would be 9.95328 Gbps−5 ppm. The 15 ppm frequency difference between the actual payload signal frequency (+10 ppm offset) and the payload envelope frequency (−5 ppm offset) is absorbed by the justification mechanism.
If the payload type is 10 GbE LAN PHY, the nominal payload frequency is 10.3125 Gbps with +/−100 ppm variation. The synchronously mapped G.709 signal would be 10.3125 times the F-OHR of 255/237 Gbps+/−100 ppm. The frequency offset is identical to the actual payload signal frequency offset. The asynchronously mapped G.709 signal would be 10.3125 times the F-OHR of 255/237 Gbps plus an offset related to the local reference frequency offset. The G.709 signal frequency with an OC192 payload type is very different from the G.709 signal with a 10GbE payload type.
It is also known in the art to convert incoming asynchronous data signals with either a higher or lower frequency than a synchronized data signal frequency by means of negative or positive bit stuffing so that frequency differences are made up, respectively, by the insertion into or removal from of spare bits or bytes in the synchronized data signal. See, for example, U.S. patent application of Rude, Publication No. 2001/0022826, published Sep. 20, 2001, now U.S. Pat. No. 6,415,006. Further, see also U.S. Pat. No. 5,757,871.
The object of this invention is to provide a plesiochronous clocking system approach that provides for the efficient transport of different client signal protocols, such as in the case of G.709 protocol or 10 GbE protocol, while rendering the network system easily compatible with all protocols and standards.